(1) Field of the Invention
The present invention relates to a method for producing a semiconductor device, more particularly to a method for producing a semiconductor device wherein, after a gate oxide layer or a capacitor oxide layer is formed, an impurity region is formed below the oxide layer.
(2) Description of the Prior Art
An impurity region may be provided below a capacitor electrode for forming a junction capacitance in a dynamic random-access memory (RAM), below a tunnel gate in an electric erasable programmable read-only memory (EEPROM), or below a gate electrode for controlling the threshold voltage in a metal-insulated semiconductor (MIS) transistor. Such a region is formed by forming a gate oxide layer or a capacitor oxide layer on the semiconductor substrate and implanting impurity ions into the substrate through the oxide layer.
When an impurity region is formed and then the oxide layer is formed, implanted impurities such as boron ions are diffused down and decrease the impurity concentration at the surface of the impurity region while impurities such as phosphorus ions or arsenic ions are piled up to increase the impurity concentration at the surface of the impurity region. Either of these phenomena results in deviations in the capacitance of a capacitor in a dynamic RAM, the amount of charge injection in an EEPROM, or the threshold voltage in a MIS transistor.
Consequently, an impurity region is usually formed by first forming a thin oxide layer acting as a capacitor oxide layer, tunnel gate oxide layer, or gate oxide layer by using a thermal oxidation process and then implanting the required impurity ions into a semiconductor substrate through the thin oxide layer.
Conventionally, the implantation of the impurity ions has been performed through the thin oxide layer from directly above using a resist layer as a mask. If the resist layer contains contaminants, the surface of the semiconductor substrate may be contaminated by the resist layer. Further, after the impurity implantation, the chemicals used to remove the resist layer contaminate or etch the thin oxide layer, thereby deteriorating the properties of the thin layer, decreasing the layer thickness thereof, and thus deteriorating the stability of the characteristics of the resultant device. Since the thin oxide layer is eventually applied with a high electric field during device operation and its dielectric characteristic is vital to the device operation, the resultant device is particularly vulnerable to the contamination of this thin oxide layer during the manufacturing process.